Hmm... I would say jail for fifteen years is far more expensive. Just think of all of the domestic care, e.g. meals etc.
For an execution, you just have to pay for any materials used (chemicals etc. in the case of lethal injection), a last meal request, a doctor to perform the administration of the chemicals and to officially certify death and maybe some of the paperwork involved would amount to a small cost. This is clearly the cheaper option.
With appeals and such, life without parole is cheaper.